5.16
Anticholinesterase Agents: Poisoning and Treatment
Anticholinesterases, also known as cholinesterase inhibitors, work by blocking the breakdown of acetylcholine, leading to its accumulation in the syna…
Anticholinesterase agents prevent acetylcholine hydrolysis to enhance cholinergic actions.
Organophosphates—a type of anticholinesterase—interact covalently with AChE, yielding an irreversible phosphorylated enzyme.
It further undergoes aging with the loss of an alkyl group, and the complex formed is resistant to hydrolytic regeneration.
Aging inhibits AChE functioning and promotes acetylcholine accumulation, potentiating cholinergic responses leading to acute toxicity.
The handling of anticholinesterases, such as insecticides, can lead to accidental poisoning, and the symptoms manifest as a result of muscarinic and nicotinic activation.
In organophosphate poisoning, cholinesterase reactivators, such as pralidoxime, are effective before the enzyme undergoes aging. Pralidoxime binds to the anionic site of the enzyme through its charged quaternary nitrogen, while the oxime end reacts with the phosphorus atom. The resulting complex diffuses away, reactivating the enzyme.
The muscarinic symptoms are managed by IV administration of atropine.
Other treatment measures include gastric lavage, maintaining blood pressure, and ensuring airway patency.
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Q1: How do anticholinesterase agents enhance cholinergic actions?
Anticholinesterase agents block the breakdown of acetylcholine by inhibiting cholinesterase enzymes, causing acetylcholine to accumulate in the synaptic cleft. This accumulation indirectly enhances both muscarinic and nicotinic actions at their respective receptors. The degree of enhancement depends on whether the agent forms reversible or irreversible bonds with the enzyme.
Q2: What is enzyme aging and why does it prevent treatment of organophosphate poisoning?
Enzyme aging occurs when the phosphorylated enzyme complex loses an alkyl group, making it resistant to hydrolytic regeneration. Once aging happens, cholinesterase reactivators like pralidoxime become ineffective because they cannot restore enzyme function. This irreversible change means treatment must occur before aging begins to successfully reactivate the enzyme.
Q3: How does pralidoxime reactivate acetylcholinesterase in organophosphate poisoning?
Pralidoxime binds to the anionic site of acetylcholinesterase through its charged quaternary nitrogen, while its oxime end reacts with the phosphorus atom of the organophosphate-enzyme complex. This interaction causes the complex to diffuse away from the enzyme, restoring its catalytic function. The reactivation is only effective before the enzyme undergoes aging.
Q4: Why can atropine treat muscarinic symptoms but not nicotinic effects in anticholinesterase poisoning?
Atropine is a cholinergic antagonist that blocks muscarinic receptors, effectively reversing muscarinic symptoms like excessive salivation and muscle contractions. However, atropine cannot reverse nicotinic effects such as muscle paralysis because it does not block nicotinic receptors. Nicotinic symptoms require cholinesterase reactivators or supportive measures like mechanical ventilation.
Q5: What are the key treatment measures for anticholinesterase poisoning?
Treatment involves multiple approaches: gastric lavage removes ingested poison, maintaining hydration and blood pressure supports vital functions, and airway patency ensures adequate respiration. Intravenous atropine manages muscarinic symptoms, while diazepam controls convulsions. In organophosphate cases, cholinesterase reactivators like pralidoxime restore neuromuscular transmission if administered before enzyme aging occurs.
Q6: What is the difference between reversible and irreversible anticholinesterase agents?
Reversible anticholinesterase agents form weak, temporary bonds with cholinesterase enzymes, allowing the enzyme to regenerate relatively quickly. Irreversible agents, such as organophosphates, form strong covalent bonds that persist much longer. Irreversible agents pose greater toxicity risk because the enzyme cannot easily recover function, leading to prolonged acetylcholine accumulation and severe cholinergic effects.
Q7: How do organophosphates cause irreversible inhibition of acetylcholinesterase?
Organophosphates interact covalently with acetylcholinesterase, forming a phosphorylated enzyme complex that is highly stable. Unlike carbamylated complexes from reversible agents, the phosphorylated complex breaks down very slowly. Over time, the complex undergoes aging by losing an alkyl group, creating a form completely resistant to hydrolytic regeneration and permanently inactivating the enzyme.